1. Field of the Invention
The invention relates to a method for the transmission of data packets from a plurality of substations to a main station via a channel they have in common, the main station transmitting a synchronizing signal which indicates the beginning of a frame which is divided into a sequence of time slots of mutually equal lengths, those substations which have a data packet ready for transmission to the main station transmitting this packet to the main station in a time slot which is randomly selected from the sequence of time slots, the main station counting the time slots which are unmutilated, empty and mutilated respectively depending on whether an unmutilated, no packet or a mutilated packet is received.
2. Description of the Prior Art
The invention also relates to a system for using this method, which in the relevant literature is usually designated "slotted ALOHA", as may be apparent from the article of reference 1. This article also describes that this known "slotted ALOHA" strategy can be used successfully only when adequate measures are taken to prevent the system from becoming unstable when there is a large supply of traffic. More particularly, such an unstable condition occurs, when there is a large supply of traffic, when the transmission of data packets from the substations to the main station becomes inefficient because the number of packets which were transmitted mutilated increases, which means an increase in the number of packets to be retransmitted which, in turn, leads to a greater inefficiency and which ultimately results in a complete blockage of the system.
Several strategies, all based on extensive theoretical considerations, have already been proposed to solve this instability problem; in this connection reference is made to references 2 to 6, inclusive. Apart from the fact that in the majority of cases no direct implementation can be derived from these publications of the control measures proposed to solve said problem, these known strategies all have the drawback that they do not take into account differences in signals received by the substations and the central station.